1. Field of the Invention
The present invention relates to a variable gain amplifier (VGA), and more particularly, to a VGA with a linear-in-dB gain.
2. Description of the Related Art
A linear-in-dB gain is required for the usage convenience of a user, when a wide dynamic range is needed like in transmitter and receiver of a CDMA system.
FIG. 1 illustrates an example of a variable gain amplifier (VGA) constructed with a conventional current-controlled variable gain amplifier and a conventional linearity-in-dB control signal converter (CSC).
Signal currents Isig+ and Isig− are input into an emitter of the current-controlled VGA constructed with Q1 to Q4. The input signal currents flow into Iout+ and Iout− due to Vy so as to form an output or flow into the power source.
Amounts of currents flowing into Iout+ and Iout− are dependent on Vy. As the amounts of the current flowing into Iout+ and Iout− increase, the gain of the VGA 120 increases. At this time, a relation between Vy and a VGA gain AI is shown in Equation 1.
                              A          I                =                                                            (                                  Iout                  +                                )                            -                              (                                  Iout                  -                                )                                                                    (                                  Isig                  +                                )                            -                              (                                  Isig                  -                                )                                              -                      1                          1              +                              exp                ⁡                                  (                                      Vy                    VT                                    )                                                                                        [                  Equation          ⁢                                          ⁢          1                ]            
As in Equation 1, only when Vy is greater than zero, the current-controlled VGA 120 has a linear-in-dB gain characteristic.
However, an area, in which a gain is large, corresponds to a case where Vy is less than zero. In this case, the current-controlled VGA loses the linear-in-dB gain characteristic. Accordingly, the current-controlled VGA is unavailable. In this case, since the current-controlled VGA is unavailable in the area in which the current-controlled VGA loses the linear-in-dB gain characteristic, the VGA has to be serially connected and used in order to compensate the lost linearity-in-dB. Accordingly, it is disadvantageous in power consumption.
In order to compensate for this, a control signal converter 110 generates VY by using an externally given gain control signal VCNT so that the VGA 120 obtains the linear-in-dB gain characteristic to the maximum gain.
A current Icnt generated in response to the gain control signal VCNT generates a voltage drop between bases of Q21 and Q20. When a current flowing through Q20 is I0, collector currents of Q10 and Q11 are respectively determined as I0exp(−gCRVCNT/VT) and I0(1−exp(−gCRVCNT/VT) due to dB characteristics of the bipolar transistors Q21 and Q20. The result Vy is represented by Equation 2.
                                                        Vy              =                                                V                                      BEQ                    ⁢                                                                                  ⁢                    11                                                  -                                  V                                      BEQ                    ⁢                                                                                  ⁢                    10                                                                                                                          =                                                V                  T                                ⁢                                  ln                  (                                                                                    I                        0                                            (                                              1                        -                                                  exp                          ⁡                                                      (                                                                                          -                                                                  g                                  C                                                                                            ⁢                                                                                                RV                                  CNT                                                                /                                                                  V                                  T                                                                                                                      )                                                                                                                                                              I                        0                                            ⁢                                              exp                        ⁡                                                  (                                                                                    -                                                              g                                C                                                                                      ⁢                                                                                          RV                                CNT                                                            /                                                              V                                T                                                                                                              )                                                                                                                                                                                            =                                                V                  T                                ⁢                                  ln                  ⁡                                      (                                                                  exp                        ⁡                                                  (                                                                                    g                              C                                                        ⁢                                                                                          RV                                CNT                                                            /                                                              V                                T                                                                                                              )                                                                    -                      1                                        )                                                                                                          [                  Equation          ⁢                                          ⁢          2                ]                                                                                    A                I                            =                              1                                  1                  +                                      exp                    ⁡                                          (                                                                        V                          y                                                VT                                            )                                                                                                                                              =                              1                                  1                  +                                      exp                    ⁡                                          (                                                                        g                          C                                                ⁢                                                                              RV                            CNT                                                    /                                                      V                            T                                                                                              )                                                        -                  1                                                                                                        =                              exp                ⁡                                  (                                                            -                                              g                        C                                                              ⁢                                                                  RV                        CNT                                            /                                              V                        T                                                                              )                                                                                        [                  Equation          ⁢                                          ⁢          3                ]            
Accordingly, a current gain AI is represented as exp(−gCRVCNT/VT) as shown in Equation 3 by substituting Equation 2 for VY of Equation 1. Accordingly, the VGA can have a linear-in-dB gain characteristic.
However, in a case where VY is generated through the method using the VGA of FIG. 1, since large number of bipolar transistors and resistors are used, the area is large. Accordingly, it is difficult to integrate the bipolar transistors and resistors. In addition, since the transistors Q20 and Q21 for generating currents which sensitively influence the gain are not designed as a differential circuit, in practice, the transistors tend to be sensitive to common noise such as a power source end. The performance deterioration due to the common noise is more serious in an integration circuit in which many elements have to be integrated into a small area.
A gain curve of the externally given gain control signal is determined by gc and R. The slope of the gain curve can be changed by externally adjusting gc and R by using switches and the like. However, when the slope is fixed, it is impossible to move the total gain upwardly or downwardly. In addition, when the VGA is used, since the gain of the VGA is different depending on components used in blocks before and behind the VGA, it is necessary to move the total gain upwardly or downwardly.